Magnetic fluids are colloidal suspensions in which magnetic particles are dispersed in a fluid. The particles are typically coated with an amphiphilic surface-active agent that prevents the particles from aggregating. Surface-active agents are typically anhydrides of fatty acids with C11-C25 acid radicals. The polar portion of the surface-active agent form a coating on the magnetic particles by covalently bonding atoms at the surface of the magnetic particles. Such bonding is termed “chemosorption.” The lipophilic portion of the surface-active agent extends radially away from the magnetic particle.
C7-C18 fatty acids are typically used as solvents for the anhydrides during the process of coating magnetic particles with surface-active agents. The fatty acids are not typically chemosorbed to the surface of the magnetic particles. The fatty acid used can optionally be a homolog of the surface-active agent.
To form the colloidal suspension, the particles are dispersed in a carrier, or dispersing fluid. The carrier fluid can be polyethylsiloxane fluids, such as those described by the general linear structural formula M2Dn, where n is 1-8, M is (C2H5)3SiO0.5, and D is (C2H5)2SiO. Alternatively, the fluid can be a synthetic olefinic polymer oil made of C5-C20 monomeric units. Combinations of polyethylsiloxane fluids and synthetic olefinic polymer oils can also be used as the carrier fluid.
The process of making an organosiloxane-based ferrofluid, such as polyethylsiloxane-based ferrofluid, is known. The process typically involves sedimentation of magnetic particles in an aqueous solution of ferric and ferrous salts, adding about 25-36% by volume of a solvent (e.g., ammonium hydroxide), rinsing the obtained sediment with acetone, followed by stabilization and peptization in a organosiloxane carrier fluid. See, e.g., Russian Patent No. RU 1621766, which is incorporated by reference in its entirety. Peptization is the process whereby sedimented magnetic particles are converted into a colloidal liquid.
Alternatively, ferrofluids can be made by sedimentation of finely dispersed magnetic particles in an aqueous solution of ferric and ferrous salts and aqueous ammonia. The obtained sediment is repeatedly rinsed with distilled water and then stabilized with a surface-active agent in an aqueous solution of acetic acid. While the mixture is warmed and stirred, the stabilized magnetic particles are first treated with a non-polar solvent, and then treated with a polar solvent. Ferrite desalination and excessive surface-active agent binding are monitored, and the product is peptized in a non-polar disperse environment. See, e.g., Russian Patent No. RU 2208584, which is incorporated by reference in its entirety.
Methods for the preparation of ferrofluids are described in U.S. Pat. No. 6,068,785, which is hereby incorporated by reference in its entirety. A number of books and references also discuss the subject of magnetic fluids, including their preparation. These include, e.g.: Magnetic Fluid Applications Handbook, B. Berkovsky, ed., Begell House, Inc., New York (1996); Ferrohydrodynamics, R. E. Rosensweig, Cambridge University Press, New York (1985); Ferromagnetic Materials—A Handbook on the Properties of Magnetically Ordered Substances, E. P. Wohlfarth, ed., Chapter 8, North-Holland Publishing Company, New York, and “Proceedings of the 7th International Conference on Magnetic Fluids,” Journal of Magnetism and Magnetic Materials, Vol. 149, Nos. 1-2 (1995), all of which are hereby incorporated by reference in their entirety.
Ideal properties of magnetic fluids vary depending upon the application in which they are used. Important characteristics to consider include the fluid's minimum viscosity, maximum flux density before saturation, uniform size of the dipoles, degree of impurities, stability of its properties over time, stability of its properties over as wide a range of temperatures as possible, non-corrosivity, low coefficient of expansion, and, for applications where optical sensing of the movement of an inertial body is used, optical transparency. Many of these properties are difficult to achieve in combination.
Moreover, magnetic fluids produced by conventional methods have several disadvantageous properties. The limitations of such fluids include a high viscosity and low magnetization saturation. Such fluids are also unstable and prone to aggregation when stored over time (i.e., have a low aggregate stability). In particular, the magnetic particles of these fluids tend to associate and clump together.
It is believed that this instability is due to molecular desorption of any coating from the magnetic particles. This primarily occurs due to desorption of the anhydride coating. Desorption of fatty acids, fatty acid micelles, anhydride micelles and/or hybrid fatty acid/anhydride micelles can also occur and contribute to the instability. Any such desorption results in the deterioration of the protective coating, or film, which prevents interparticle aggregation. Desorption of the surface-active agents also results in the formation of surface-active agent micelles, as shown in FIG. 1B. Such micelles are aggregates of amphiphilic surface-active agent molecules (FIG. 1A) whose polar groups form the micelle core surrounded by a layer of non-polar hydrocarbon radicals. Cohesion forces among such surface-active agent molecules are believed to be due to dipole-dipole interaction between ion pairs and possible hydrogen bonds. The extent of coating can be determined based on viscosity characteristics of the magnetic fluid.
During the manufacture of the magnetic particles, solubilization of the surface-active agent anhydrides by the fatty acid solvent further exacerbates the instability of conventional magnetic fluids. Such solubilization reduces the ability of surface-active agent molecules to coat the magnetic particles. For example, where surface-active agent micelles have formed, fatty acid solvent molecules wedge between surface-active agent molecules, so that their polar groups face the micelle core, and their non-polar parts align in parallel to hydrocarbon radicals of the surface-active agent, as in FIG. 2B. Hybrid micelles composed of surface-active agent and solvent molecules are formed (FIG. 3), grow larger in size and become bulkier. Additional energy is needed for a surface-active agent molecule to escape from such a hybrid micelle, and is thus less likely to form a coating on a magnetic particle.
Solubilized surface-active agent micelles or hybrid micelles also create the potential for undesired physical adsorption to occur at the magnetic particle surfaces. Since chemosorption of individual surface-active agent molecules is complicated, physical adsorption of surface-active agent micelles, as shown in FIG. 4, is possible. Physical adsorption of surface-active agent micelles or hybrid micelles onto magnetic particle surfaces happens due to Van der Waals forces. In contrast to chemosorption, physical adsorption does not result in the formation of covalent bonds. Physical adsorption is a problem both during the manufacture of the magnetic fluid as well as during its storage. Such intermolecular forces have an action range from about 1-100 nm, have unsaturated interionic bonds, and bond energies of about 10-40 kJ/mol. Because the bond energy of physical adsorption is considerably smaller than that of chemosorption (chemosorption bond energy can reach 400 kJ/mol), micelles adsorbed to the surface of magnetic particles are easily desorbed. Such desorption further contributes to the low aggregational stability in that the particles are prone to losing their coatings.
Whether by formation of micelles or solubilization, desorption of surface-active agent molecules leads to reduction of the magnetic particles' protective film or coating. As the particles lose their coating of surface-active agent molecules, they become more prone to approach one another and aggregate, as depicted in FIG. 5. Colloidal suspensions of such particles are very unstable, inhomogeneous, and have low magnetic flux saturation values.
Accordingly, there is a need in the art for colloidal magnetic fluids that have high aggregational stability (i.e., magnetic fluids that do not form aggregates). The present approach addresses these problems by describing a method of manufacturing magnetic fluids that allows better surface-active agent adsorption on magnetic particles. Such particles have denser protective coats, or films, and provide colloidal suspensions having greater aggregational stability.